Lithographic printing plates (after process) generally consist of ink-receptive areas (image areas) and ink-repelling areas (non-image areas). During printing operation, an ink is preferentially received in the image areas, not in the non-image areas, and then transferred to the surface of a material upon which the image is to be produced. Commonly the ink is transferred to an intermediate material called printing blanket, which in turn transfers the ink to the surface of the material upon which the image is to be produced.
At the present time, lithographic printing plates (processed) are generally prepared from lithographic printing plate precursors (also commonly called lithographic printing plates) comprising a substrate and a radiation-sensitive coating deposited on the substrate, the substrate and the radiation-sensitive coating having opposite surface properties. The radiation-sensitive coating is usually a photosensitive material, which solubilizes or hardens upon exposure to an actinic radiation, optionally with further post-exposure overall treatment. In positive-working systems, the exposed areas become more soluble and can be developed to reveal the underneath substrate. In negative-working systems, the exposed areas become hardened and the non-exposed areas can be developed to reveal the underneath substrate. Conventionally, the actinic radiation is from a lamp (usually an ultraviolet lamp) and the image pattern is generally determined by a photomask which is placed between the light source and the plate. With the advance of laser and computer technologies, laser sources have been increasingly used to directly expose a printing plate which is sensitized to a corresponding laser wavelength; photomask is unnecessary in this case.
The radiation-sensitive layer is generally coated onto a smooth or grained substrate at sufficient thickness to form a flat, smooth surface. While a plate with a flat, smooth radiation-sensitive layer are very useful, it often suffers from the problem that the radiation-sensitive layer surface tends to block to the back of another plate at extreme environmental condition, such as higher temperature, higher pressure, and higher humidity. Also, such a plate can suffer from higher tackiness when the radiation-sensitive layer is formulated with higher content of liquid components (such as monomers) or at higher humidity. Coating the radiation-sensitive layer on a grained substrate at a thin coverage so that the radiation-sensitive layer surface is below the top of the microscopic peaks of the grained substrate surface has been proposed in the patent literature; however, such a plate suffers from poor ink receptivity in the image areas and poor press durability due to incomplete coverage of the microscopic surface by the radiation-sensitive layer. Therefore, there is a desire for a lithographic plate which has excellent block resistance, non-tackiness, ink receptivity, and press durability.
Currently, most commercial lithographic plates require a development process after the plates being exposed and before put on press. A liquid developer is used to dissolve and clean off the non-exposed areas (for negative plate) or the exposed areas (for positive plates). Such a development process is time and labor consuming and generates wet waste. It would be desirable that such a tedious development process can be eliminated.
On-press developable lithographic printing plates have been disclosed in the literature. Such plates can be directly mounted on press after exposure to develop with ink and/or fountain solution during the initial prints and then to print out regular printed sheets. No separate development process before mounting on press is needed. Among the on-press developable lithographic printing plates are U.S. Pat. Nos. 5,258,263, 5,407,764, 5,516,620, 5,561,029, 5,616,449, 5,677,110, 5,811,220, and 6,014,929. An on-press developable lithographic plate generally comprises, at least, a substrate and a radiation-sensitive layer. In order for an on-press developable plate to be useful, the non-hardened (for negative working plate) or the solubilized (for positive working plate) areas should be able to be cleaned off completely on press with ink (for waterless plate) or with ink and/or fountain solution (for wet plate) during the initial press operation. Acceptable printed sheets should be achieved within a few impressions. Therefore, the non-hardened or solubilized areas of the radiation-sensitive layer should be able to be penetrated, softened, and dispersed or dissolved by ink and/or fountain solution within seconds; the softened, and dispersed or dissolved areas of the radiation-sensitive layer will be absorbed by ink and/or fountain solution, and/or removed by the press offset roller and printing papers. Considering the limited amount of ink or fountain solution on a printing press and the high viscosity of the ink, it is very difficult to obtain a plate which is non-tacky and can be developed on press quickly and cleanly. Radiation-sensitive layer with fast ink and/or fountain solution penetrability and developability usually has poor non-tackiness and poor block resistance, compared to conventional plates. Here, block resistance is defined as the capability to resist the radiation-sensitive layer from transferring to the back of another plate when stacking many plates together. Therefore, there is a desire for an on-press developable lithographic printing plate with excellent non-tackiness, block resistance, on-press developability, and press durability.
Various laser ablatable plates have been described in the literature. Examples of such plates include U.S. Pat. Nos. 4,054,094, 5,605,780, 5,310,869, and 5,493,971. Such plates comprise, at least, a substrate and one or more laser ablatable radiation-sensitive layers on the substrate. During the imagewise exposure, infrared laser thermally ablates the radiation-sensitive layer (or layers) in the exposed areas to reveal the substrate, forming a plate consisting of substrate areas and coating areas in an imagewise distribution. Usually, a coating with the best ablation capability (requiring the least laser energy to achieve complete ablation) does not have the best block resistance, and a coating with best block resistance does not have the optimum ablation capability. There is a challenge to have both good ablation capability and block resistance.
I have found that a lithographic plate comprising on a roughened substrate a substantially conformally coated radiation-sensitive layer can provide no or low tackiness and excellent block resistance, while allowing excellent press durability. For on-press developable lithographic plate, such a radiation-sensitive layer configuration also allows excellent on-press developability. For laser ablatable lithographic plate, such a radiation-sensitive layer configuration also allows excellent ablatability. The radiation-sensitive layer is substantially conformally coated on the roughened substrate surface in a way so that the surface of the radiation-sensitive layer has peaks and valleys substantially corresponding to the major peaks and valleys of the substrate microscopic surface. It is very surprising that such a plate surface gives very low tackiness and excellent block resistance even with a radiation-sensitive layer which is very tacky and has poor block resistance when coated to form a smooth surface. It is also very surprising that such a thin coating (especially in the peak areas) can still provide excellent press durability.